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I've been kind of struggling to wrap my head entirely around trans-critical CO2 refrigeration.

It's a weird system."

Maybe this will help JP - These three articles are from ACHR News back in 2003 - 2004 on Transcritical and Subcritical refrigeration that explains it pretty well.

The first one explains the cycle and the last two are examples of installations in a supermarket in Copenhagen, Denmark in the first one(pretty interesting), and system compatibility and safety in the second one.

How is any refrigeration being done? Since the CO2 is hitting the HPEV as a supercritical fluid, it isn't truly a liquid and, thus, isn't changing state as we're accustomed to seeing.

Without a change of state, you're not going to be absorbing heat and, therefore, you can't be refrigerating.

Start at the beginning, the gas cooler (or just think of it as a condenser), is still giving its energy away. Because the gas is trans-critical, the molecules are just not sticking together as in a liquid. Even though it has the energy as if it were a liquid.

When the trans-critical fluid (i use fluid, not the word liquid), hits the expansion device (pressure drops, below the trans- critical point. As with a normal refrigerant, part of the fluid reverts to liquid and the remainder reverts to a vapour. The colder the cooled trans-critical fluid (like sub cooling), the less flash gas you have after the expansion device, and the greater the liquid and more net refrigeration effect.

You may find, that the CO2 which is delivered to the cases, is not trans-critical, (as change as happened at a the second stage of compression)

Have you ever seen cold after your nitrogen valve, when purging, another trans-critical fluid. Look up Joule Thompson.

"Figure 3 represents a transcritical cycle with CO2. The cycle diagram looks the same; the only difference is that the heat rejection process occurs above the critical point. Unlike the subcritical condensing process, where temperature stays constant, temperature decreases during the entire transcritical heat rejection process. There is no condensation in a transcritical cycle, and we call the process&#160;gas cooling. In low ambient conditions, the process might occur below the critical point, and condensation, rather than gas cooling, would occur. This is the fundamental difference between subcritical and transcritical cycles"

"Figure 3 represents a transcritical cycle with CO2. The cycle diagram looks the same; the only difference is that the heat rejection process occurs above the critical point. Unlike the subcritical condensing process, where temperature stays constant, temperature decreases during the entire transcritical heat rejection process. There is no condensation in a transcritical cycle, and we call the process&#160;gas cooling. In low ambient conditions, the process might occur below the critical point, and condensation, rather than gas cooling, would occur. This is the fundamental difference between subcritical and transcritical cycles"

I had understood that the heat rejection took place above the critical point, meaning that the fluid didn't truly condenser. Heat rejection wasn't the problem for me.

Originally Posted by barbar

Start at the beginning, the gas cooler (or just think of it as a condenser), is still giving its energy away. Because the gas is trans-critical, the molecules are just not sticking together as in a liquid. Even though it has the energy as if it were a liquid.

When the trans-critical fluid (i use fluid, not the word liquid), hits the expansion device (pressure drops, below the trans- critical point. As with a normal refrigerant, part of the fluid reverts to liquid and the remainder reverts to a vapour. The colder the cooled trans-critical fluid (like sub cooling), the less flash gas you have after the expansion device, and the greater the liquid and more net refrigeration effect.

You may find, that the CO2 which is delivered to the cases, is not trans-critical, (as change as happened at a the second stage of compression)

Have you ever seen cold after your nitrogen valve, when purging, another trans-critical fluid. Look up Joule Thompson.

I think that was the thing that I was missing, somehow. That the supercritical fluid essentially had the same properties as a liquid is so far as the refrigeration system was concerned.

I couldn't quite wrap my head around refrigeration happening without liquid hitting the metering device.

Barbar, you actually brought up a good example, your nitrogen tank gets cold as you release vapor from the regulator. That nitrogen starts as a high pressure gas (not a liquid) and absorbs heat as it drops in pressure to low pressure gas exiting the reg. I think that is a perfect example of the Joule Thomson effect.

Is this a conventional supermarket or cold storage warehouse? The upfront cost must have been substantial compared to a conventional refer system. So this company must be planning for the long haul or going green or new tech is part of their marketing/ image strategy.

It's amazing how much of an investment has been made to get the Valve Core Gripper off the ground. Im not in the green yet but I am climbing out of the hole. All the profit I make I reinvest back into the tool. Currently working on the drawings to get a production run of the Enviro-Gripper going. Hope to get it out by the end of the year. Between my job and chasing my dreams I'm working 24hrs a day! Whats your question zartangreen?

Rusty Walker from Hill Phoenix just did our training class last week. Rusty did a great job as I know pretty much know whats going on now. Now that I know more about how it works I am beginning the piping to the warehouse coolers and freezers. I will get some pictures today and review with you how the system works.

Racks are in place. We have 4 systems. Now that I had the training class, I'm a lot clearer on how these things will function. I am honored and privilaged to have this opportunity and would like to thank my boss and his family for everything they have done for me. My batteries are all charged up for this install! I have started working 7 days a week! I love my family. My wife is very understanding and supports me in every way imaginable. Right now I'm really loving my job. So much so, Ive had zero opportunities to work on getting the Valve Core Grippers into the supply houses. Yet another goal of mine for the year. My New Years resolution was to get more organized. 2013, the year of the Organization!

The warehousing evaporators are hung and the "Gas Coolers" are on the roof. Midwest helicopters put them up there for us. The condenders are called gas coolers because on the hot days of the year the CO2 discharge vapor is just cooled and no change of state occurs. In temps less than 87 Deg.F. change of state does occur in the "Gas Coolers". The coolers will be piped in Sch.80 SS pipe and will be tig welded while purging with Argon.